FIG. 1 is a simplified block diagram showing an ER transmitter (TX) 1 architecture that includes an amplitude modulation (AM) chain and a phase modulation (PM) chain. Bits to be transmitted are input to a bits to polar converter 2 that outputs an amplitude signal to an amplitude modulator (AM) 4. The AM 4 (after digital to analog conversion) supplies a signal for controlling the output level of a TX power amplifier (PA) 6 through the use of a controllable power supply 5. The bits to polar converter 2 also outputs a phase signal to a frequency modulator (FM) 7, which in turn outputs a signal via a phase locked loop (PLL) to the input of the PA 6. The transmitted signal at an antenna 9 is thus generated by simultaneously using both phase and amplitude components. The benefits that can be gained by using the ER transmitter architecture include a smaller size and an improved efficiency.
However, it has been recognized that the ER-based system is sensitive to the propagation time difference between the AM and PM chains. The existence of the propagation time difference generates undesired effects in the ER transmitter 1. For example, the adjacent channel leakage ratio (ACLR) level at the TX antenna 9 may exceed the acceptable levels required by cellular system specifications, where ACLR defines the ratio between the transmitted channel signal in-band signal and the level of the leaking interference signal into neighboring channels. Furthermore, the existence of the propagation time difference can result in an increase in the error vector magnitude (EVM), which is manifested at the receiver as an increase in the bit error ratio (BER), which can be expressed as the number of erroneously received bits divided by the total number of received bits. The EVM results from the fact that a received symbol may not correspond precisely to an ideal symbol shown in a constellation diagram due to noise associated with the communication channel and imperfections of both the transmitter and receiver. The difference between a received symbol and an ideal symbol can be represented as an error vector (EV). Generally, the smaller the magnitude of the error vector, the better the performance of the communication system. The EVM is the root mean square (RMS) value of the error vector over time at the precise time instant of the symbol clock transitions. EVM is typically normalized to either the amplitude of the outermost symbol, or the square root of the average symbol power.
Each symbol may be represented as a particular amplitude and phase. Thus, the transmitted signal may vary in amplitude and/or phase to transmit a string of consecutive symbols, and the amplitude and phase components of a signal may be processed separately in a transmitter (as shown in FIG. 1). The EVM for a communication system can vary based on the delay applied between the amplitude of the transmitted signal and the phase of the transmitted signal.
Justice et al., in U.S. Patent Application Publication U.S. 2002/0168020 A1, describe the use of a processor with a delay adjustment module to adjust the delay between the amplitude and phase components of the signal to be transmitted. This is done based on the transmit power of the information signal. In practice, the delay value is selected to minimize the combination of the EVM and the ACPR (Adjacent Channel Power Ratio).
With regard to the delays between the AM and PM chains, it can be shown that the required time resolution is 1/128 of the symbol time in the EDGE (Enhanced Data rate for Global Evolution) type cellular system, and 1/64 of the symbol time in the WCDMA (Wideband Code Division, Multiple Access) type cellular system. For example, for the WCDMA system the time resolution is about 4.07 ns, a value that is too small to be directly measured in a practical manner within the mobile station.